Data from: Trait similarity among dominant of highly-competitive species rather than diversity increases productivity in semi-arid Mediterranean forests

Biodiversity plays a pivotal role in forest productivity and, through it, on the provision of ecosystem services. Consequently, understanding the mechanisms by which biodiversity drives productivity is a challenge for sustainable forest management. It has been proposed that functional dissimilarity can increase forest productivity, particularly in water-limited ecosystems (as Mediterranean forests), where water stress might increase complementarity resource use among species (hypothesis of niche complementarity). Alternatively, strongly water-limited forests may favor the dominance of woody species with slow-growth and conservative resource-use trait values, which in turn, might increase plant community productivity proportionally to their biomass (hypothesis of mass-ratio). The effects of such mechanisms on productivity is yet understudied in water-limited forests. We assessed the effect of both previously described mechanisms on productivity in a semi-arid Mediterranean-type climate matorral of Chile through a trait-based approach. We installed twenty-three 25 × 25 m plots in the matorral of the National Reserve of “Roblería del Cobre de Loncha” located in the Coastal Range. Forest productivity was estimated as above-ground biomass changes between 2010 and 2017. The mean species pairwise dissimilarity (MPD) and the community-weighted mean trait values (CWM) were calculated for each single trait as surrogates of the hypotheses of niche complementarity and mass-ratio, respectively. Environmental variables (e.g. altitude, slope, precipitation) were included as potential drivers influencing productivity. We performed a selection procedure of multiple linear regression models based on the Akaike Information Criterion. Our findings show that higher forest productivity was related with higher trait similarity of plant maximum height and onset of growth among species. Besides, productivity increased with CWM values that reflect greater dominance of species exhibiting acquisitive traits (i.e. greater maximum plant height, lower wood density and earlier onset of growth) rather than conservative traits. Moreover, forest productivity increased with increasing altitude, precipitation and decreasing temperatures from 300 to 900 m. Therefore, in contrast with our hypotheses, productivity increased by greater trait similarity among dominant highly-competitive species - which might be favored by environmental conditions at middle latitudes of the Coastal Range. The sustainable management of forests with the main focus on productivity should promote higher dominance of highly-competitive species with similar functional trait values in mesic conditions and greater trait functional dissimilarity in stressful environments. to increase drought resistance.

生物多样性（biodiversity）对森林生产力起着核心支撑作用，并通过这一过程影响生态系统服务（ecosystem services）的供给。因此，阐明生物多样性驱动生产力的作用机制，是可持续森林管理（sustainable forest management）亟待攻克的挑战。有研究提出，功能分异（functional dissimilarity）可提升森林生产力，尤其在水分限制生态系统（如地中海型森林（Mediterranean forests））中——此时水分胁迫（water stress）可能强化物种间的资源利用互补性，即生态位互补假说（niche complementarity hypothesis）。与之相对，重度水分限制的森林可能更倾向于由生长缓慢、资源利用保守的木本物种占据优势，这类物种的生物量占比提升可按比例提高植物群落生产力，即质量比假说（mass-ratio hypothesis）。目前，这类机制对水分限制森林生产力的影响仍有待深入探究。我们基于性状分析方法，在智利一处半干旱地中海型气候灌丛（matorral）中评估了上述两种机制对生产力的影响。研究团队在位于海岸山脉的“隆查铜橡树国家保护区（Roblería del Cobre de Loncha National Reserve）”的灌丛内设置了23个25×25米的样地。森林生产力以2010年至2017年的地上生物量（above-ground biomass）变化量作为估算指标。我们分别计算了各单个性状的物种平均两两分异度（mean species pairwise dissimilarity，MPD）与群落加权平均性状值（community-weighted mean trait values，CWM），以分别作为生态位互补假说与质量比假说的替代衡量指标。同时将海拔、坡度、降水量等环境变量纳入考量，作为影响生产力的潜在驱动因子。基于赤池信息准则（Akaike Information Criterion），我们开展了多元线性回归模型的筛选流程。研究结果显示，更高的森林生产力与物种间植物最大高度、生长始期的性状相似性提升显著相关。此外，生产力随群落加权平均性状值的升高而增加，这类性状值反映了具备获取型性状（acquisitive traits，即更大的最大株高、更低的木材密度与更早的生长始期）的物种优势度更高，而非保守型性状（conservative traits）的物种。进一步分析表明，在300米至900米的海拔区间内，森林生产力随海拔升高、降水量增加与气温降低而提升。因此，与我们的初始假说相悖，生产力随优势竞争力物种间更高的性状相似性而提升——这类高竞争优势物种可能受益于海岸山脉中纬度区域的环境条件。若以生产力为核心目标开展森林可持续管理，则应在湿润生境（mesic conditions）中促进具备相似功能性状值的高竞争力物种占据更高优势度，而在胁迫环境（stressful environments）中则应提升性状功能分异程度，以增强抗旱性（drought resistance）。